PARIS — In a year when the extremes were supposed to be on the rise in France, the unpopular leftist president has decided not to run for re-election, the onetime conservative favorite has been engulfed by scandal and Emmanuel Macron, a pro-market centrist and political novice, has unexpectedly become the man to beat.

Mr. Macron has edged ahead of conservative candidate Francois Fillon in polls with 23 percent support, trailing only far-right National Front candidate Marine Le Pen in one of the most turbulent periods in French politics in memory.